iio: dac: adi-axi-dac: fix bus read

Fix bus read function.

Testing the driver, on a random basis, wrong reads was detected, mainly
by a wrong DAC chip ID read at first boot.
Before reading the expected value from the AXI regmap, need always to
wait for busy flag to be cleared.

Fixes: e61d717 ("iio: dac: adi-axi-dac: extend features")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250409-ad3552r-fix-bus-read-v2-1-34d3b21e8ca0@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: spectrum70

drivers/iio/dac/adi-axi-dac.c

@@ -707,6 +707,7 @@ static int axi_dac_bus_reg_read(struct iio_backend *back, u32 reg, u32 *val,
 {
 	struct axi_dac_state *st = iio_backend_get_priv(back);
 	int ret;
+	u32 ival;
 
 	guard(mutex)(&st->lock);
 
@@ -719,6 +720,13 @@ static int axi_dac_bus_reg_read(struct iio_backend *back, u32 reg, u32 *val,
 	if (ret)
 		return ret;
 
+	ret = regmap_read_poll_timeout(st->regmap,
+				AXI_DAC_UI_STATUS_REG, ival,
+				FIELD_GET(AXI_DAC_UI_STATUS_IF_BUSY, ival) == 0,
+				10, 100 * KILO);
+	if (ret)
+		return ret;
+
 	return regmap_read(st->regmap, AXI_DAC_CUSTOM_RD_REG, val);
 }